The α-Gal (Galα1,3Galβ1,4GlcNAcβ1-R) carbohydrate epitope is expressed throughout the animal kingdom, with the notable exception of Old World monkeys, apes and humans. These species lack the epitope because of an inactivating mutation in the gene encoding the α1,3 galactosyltransferase responsible for its biosynthesis. In animals where it is present, it is carried both by lipids and on a variety of carrier proteins. Species lacking the epitope, e.g. humans, have antibodies recognizing α-Gal, possibly as a result of an immune response against bacterial capsular polysaccharides of the intestinal microflora. Anti-α-Gal (anti-Gal) antibodies are of all antibody classes and subclasses, and as much as 1% of human IgG has this specificity.
Mucins such as MUC1, and mucin-like molecules with highly O-glycosylated domains, such as P-selectin glycoprotein ligand-1 (PSGL-1), are extensively glycosylated high molecular weight (>200 kD) proteins and are targets for the α1,3 galactosyltransferase. Mucins are abundantly expressed in normal cells such as leukocytes and in many human cancers of epithelial origin.